Distributed Localized Shape Control of Gossamer Space Structures

Future earth science, space science, exploration, and reconnaissance space missions will require increasingly large and lightweight apertures. Although they have low areal mass density, the deployed aperture structures must capture and hold a surface figure to a fraction of a wavelength in the presence of thermal, slew, and vibration disturbances. Active control of surface figure is a key technology for the success of gossamer space structures. Less than several hundred actuators could be still controlled by using a centralized computing element. If, however, there are thousands of actuators distributed in the surface, the control hardware and computations should be distributed as well. This paper discusses how an efficient control of a gossamer structure shape can be achieved using large distributed actuator arrays. Advanced algorithms using only local information about errors and actuation for collocated and neighboring positions in each of the distributed computational elements allow achieving required control performance. A gossamer structure with built-in distributed actuators, sensors, and computational elements can be made scalable to a very large size. Of course, integrating thousands of actuators, in a structure in a practically affordable way requires actuators are mass producible. MEMS technologies based on electrostatic actuation and implemented on compliant plastic substrates, represent a highly attractive proposition thanks to their very low areal density. A distributed surface control approach is a key enabler for future gossamer space apertures. Gossamer Space Apertures To meet the future mission demands for large lightweight space apertures, gossamer structures will be required with ever decreasing areal densities [1,2]. At first these will be accomplished with “rigid” deployable systems, followed by larger, more flexible deployables, shell structures, inflatables, and membranes. The surface precision of these radio frequency (RF) and optical (IR/visible/UV) reflectors will remain at a fraction of the wavelength regardless of dimension of the aperture. To take advantage of the ability to collect low signals, one requires large areas; to get high resolution, one requires large aperture dimensions (or baselines for sparse aperture instruments). An example large RF aperture is the 5 meter diameter TDRSS mesh deployed antenna with a surface precision of half a millimeter and mass of 24 kg. In the optical, it is the 2.4 meter Hubble Space Telescope (HST) primary mirror with an effective surface precision (after corrective optics) of 20 nanometers and mass of about 400 kg. HST corrects only for piston, tip and tilt (3 actuators). In the next decade, the Next Generation Space Telescope (NGST) will be designed to use deployed panels and perhaps hundreds of actuators to control a mirror surface to about 50 nanometers across its 8 meter diameter with an areal density of 10 kg/m. Figure 1: Number of actuators required to hold surface precision versus aperture size and areal density (with several known or predicted point designs plotted). A Gossmaer structure such as a 200 kg, 50 meter IR reflector would require millions of actuators. Future space apertures will be increasingly called upon to provide diffraction limited surface figures at ever increasing dimensions. Large, lightweight (gossamer) apertures of future space missions will not meet there figure precision passively upon deployment due to thermal effects, gravity unloading, materials uncertainty, and mechanical precision AIAA-2001-1197 Copyright C 2001 by Honeywell Inc. Published by the American Institute of Aeronautics and Astronautics, Inc. with Permission AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Seattle, WA, April 2001